Turbulence is not going to be the main downwind problem, it will be giant rotators.

Physics requires that angular momentum be conserved. It takes a huge amount of torque to turn a 3 MW turbine, and the angular momentum that is imparted to the turbine will result in an equal and opposite angular momentum being imparted to the wind. This shows up as a giant rotating column of air.

That downwind rotator will dissipate but the interesting question is how far downwind does it take to completely homogenize the velocity field so that it is all uniform again? And then of course what is the net decrease in the uniform downwind velocity.

Looking at the picture from post #1, it looks like dissipation will not occur for a mile or more.

When I plug in the numbers provided earlier (45m blade length, 3MW turbine), I get the following:
Assuming an upwind velocity of column of air hitting the cross-section of the blade area = about 24 mph
Downwind velocity of the column of air exiting the area = about 15 mph.

But that is just the column of air. When it mixes back in to the surrounding undisturbed air, the net downwind velocity will be somewhere between 15 and 24. It will depend on density of turbines, how far downstream, etc.

To many generalizations in this discussion about the effect of windmills.
Of course they affect the wind.

But, it seems to me that the starting point must be wind direction at the windmills and their relation to the river.
SW will be different than W or NW.
Pretty basic to understand.
Next is to determine if the prevailing wind direction has changed in the area.
Once you know those facts then a windmill discussion might make sense.
The causes for light wind might be from other causes such as where the high and low pressures sit, or the depth of marine surges.
I find it less than a coincidence that the thinner marine layers on the coast, that we have seen for a couple of years, happens at the same time that windmills mess with the wind.

Using this windmill conversation I suppose you could make a case that the windmills are cause the reduced ocean upwelling.

Thing is, this is not an 'either/or' kind of scenario, but more like 'both/and'. Windmills *&^%$ the wind downstream. Back when we had big fat marine layers and southwest more often than not, nobody would have cared. Even now, on those days when it swings south, so the wind on the river is more in line with the river, windmills are just part of the scenery.

Instead, near as I can tell, the wind has generally:

a.) gotten lighter, and
b.) swung more west/northwest, which
c.) puts the stretch of river from The Sign to Rio Vista downwind of these things,

which means, more often than not

d.) you can easily knock 10mph off the wind on the river.

What's worse, is that the variance increases dramatically as an inverse of the windspeed, when it is downwind of the turbines, which not only makes it hole-y, but screws up the texture { much greater variance in the wave size and direction } big time.

<You can see this> just standing on the beach on a WNW day...

So, pretty much, Sherman is mostly toast east of The Sign, most of the time { don't take my word for it -- there is never a soul at Power Lines North, which, this time of year, used to be standing room only }, and now the water people want to close the road west of it.

Until the climate swings back { El Nino? } to 'the way it was, back in the day', I think the case can be made that unless it's southwest at 25, which happens maybe half-a-dozen times a year now?, you can stick a fork in this place.

Too bad we can't get a class-action suit out it. Or a free trip to Baja or something.

I don't know about making the simple judgment of why it happens but the simple definite impact is that 80% of the time the wind and just as importantly the water conditions are hosed. If you want some long term perspective there several of us you could talk to, some who you sailed at glass beach with, who have been living and sailing out of Lelia St for 30 years.

windfind wrote:

Hi Gang,

I am slowly working on a blog about turbines and Sherman Island winds. There is no easy answer to the question but the amount of downwind impact of turbines depends upon the wind direction, wind speed and the stability of the near surface atmosphere.

Plus in the last 20 years the average marine layer depth has decreased a lot which impacts the wind distribution and strength.

All of which makes it difficult to make a simple judgement about the impact turbines on the winds near the Sign.

Mike F. Godsey; While I absolutely think there are very important local effects that affect the wind, the building of the doubled sized lot at the Larkspur GG Ferry landing, other building there and especially the windmills growing like invasive weeds up wind of the Sacramento river, and (I'm running out of breath now) the possible building upwind of Coyote or the major expansion of SFO etc (whew!), the memories of old timers are sketchy at best. In medicine a retrospective memory based study is considered seriously flawed if not useless research. In medicine, prospective double-blinded crossmatch-controlled studies are what are considered reliable. They take years and lots of time, effort and money to do. Another very important factor in global warming research and thus, wind studies going back, are urban island effects (E.G. the doubling of the parking lot in Larkspur in 2008).

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